What if you couldn't force yourself into your car to drive across town or across a bridge? If the very sight of a spider, a snake, or even a clown at a child's birthday party overwhelmed you with fear?

For the millions of people who suffer from phobias, the ordinary can become unbearable.

Phobias may arise from childhood psychological trauma, abnormalities in the structure of the brain, and even human evolution.

The Function of Fear

"Fear is actually there to protect us. It's unpleasant when we experience it, but the whole fear response is designed to protect us from danger," said Martin Antony, a psychologist at St. Joseph's Hospital in Hamilton, Canada.

"When we're afraid," he added, "our heart races to get blood to the parts [of the body] that need it. We breathe more heavily to get oxygen to the big muscles, we sweat to cool off the body so we can perform more effectively."

Phobics, however, feel intense fear even when there is no real danger. The fear can be so bad that it disrupts their lives.

Phobia, a new series from the National Geographic Channel, explores the origins and treatment of extreme fears. The series looks at the problem from the phobic's point of view, revealing how a person's life can be dramatically changedand even controlledby his or her fear.